The etiology of myopia is still unclear, but it is believed to be a failure of the emmetropization mechanism, a mechanism that alters the eye's growth rate after birth to bring the image into focus on the retina. When the emmetropization process fails, and the eye grows too big, myopia occurs. The results of these experiments are fundamental to understanding how temporal changes in the visual environment interact with visual sensitivity, and how this affects eye growth, and will ultimately pave the way to the development of visual environments that prevent myopia development. The hypothesis is that eye growth depends on the relative cone contrast sensitivity of the long-, middle-, and short-wavelength sensitive cones (S-, M-, and L- cones)-and hence to the temporally sensitive, color and luminance contrast signals. Increased relative cone contrast sensitivity of the L- and M- cones, and/or the absence of contrast at short-wavelengths, and low temporal frequencies, would be expected to make the eye more myopic as the eye compensates preferentially for the focal plane of longer wavelengths. Specific Aims 1 and 2 aim to determine how relative cone sensitivity and temporal sensitivity affects emmetropization in the chick. Chicks will be binocularly exposed to white light modulated either in color, or luminance (Mean level 650 lux), with or without blue light. First, they will be exposed at a range of temporal frequencies (0, 0.2, 1, 5, 10 Hz) for three days. Then, chicks will be binocularly exposed to color and luminance modulated stimuli, of varying cone contrast ratios comparing either L- and M-cones, or S- and LM-cones, at three temporal frequencies (0.2, 2, 10 Hz) for three days. Measurements of the change in ocular components will be made be made with a non-contact ocular biometer (Lenstar LS 900) and Hartinger Refractometer. The analysis will compare the change in ocular components with frequency and illumination condition, and the weighting of the resultant luminance and color signals. Specific Aim 3 aims to determine if cone contrast sensitivity is related to myopia development in humans. Two types of test stimuli will be used: (A) A 3 cycle per degree, horizontal Gabor patch, in sine phase (2? standard deviation) modulated along four different directions in cone contrast space (B) a Gaussian Blob, with ?=2?. This blurred spot has a single contrast polarity, unlike the Gabor patches, and thus can be used to test increment (S+) and decrement (S-) stimuli separately. In addition to measuring simple detection thresholds, the S+ and S- thresholds will be re-measured in the presence of S-cone masking noise. Ninety seconds of adaptation to the gray background screen precedes each run of 100, 2-alternative forced choice trials. Data are fit by the Weibull psychometric function, with threshold determined as the contrast corresponding to 82% correct. Cone sensitivity will be described relative to an individual's sensitivity to luminance contrast.